Bump tensorflow from 1.12.0 to 1.15.2

Bumps [tensorflow](https://github.com/tensorflow/tensorflow) from 1.12.0 to 1.15.2.
- [Release notes](https://github.com/tensorflow/tensorflow/releases)
- [Changelog](https://github.com/tensorflow/tensorflow/blob/master/RELEASE.md)
- [Commits](tensorflow/tensorflow@v1.12.0...v1.15.2)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

requirements.txt

@@ -24,7 +24,7 @@ scipy==1.2.0
 six==1.12.0
 sklearn==0.0
 tensorboard==1.12.2
-tensorflow==1.12.0
+tensorflow==1.15.2
 termcolor==1.1.0
 tqdm==4.29.0
 Werkzeug==0.14.1